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"The tone and strength of the message were discussed beforehand," Sanders said

Trump has long been bothered by North Korean Leader Kim Jong Un's threats

Bridgewater, New Jersey (CNN)President Trump's fierce warning to North Korea was improvised in the moment, not part of a scripted statement that he prepared to deliver Tuesday, according to three people with knowledge of the remarks.

Trump has long been bothered by North Korean Leader Kim Jong Un's threats against the United States and has privately vented about a powerful United States response to the rogue nation. His statement on Tuesday, flanked by top White House aides and his new chief of staff Gen. John Kelly, mimics those private remarks, a source said.

"North Korea best not make any more threats to the United States," Trump said during a meeting on opioids from his golf club in New Jersey. "They will be met with fire and fury like the world has never seen ... he has been very threatening beyond a normal state. They will be met with fire, fury and frankly power the likes of which this world has never seen before."

Trump's comment, which came shortly before North Korean officials threatened the United States territory of Guam with missiles, was criticized as too bellicose and direct by national security and defense experts who argued any conflict involving nuclear weapons called for calm and reserve.

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"I take exception to the President's comments because you've got to be sure that you can do what you say you're going to do," Sen. John McCain said in an interview with Phoenix radio station KTAR. "The great leaders I've seen don't threaten unless they're ready to act and I'm not sure President Trump is ready to act."

White House press secretary Sarah Huckabee Sanders said in a statement that Kelly and members of Trump's national security team "were well aware of the tone of the statement of the President prior to delivery. The words were his own."

"The tone and strength of the message were discussed beforehand," Sanders said. "(Gen. Kelly and others) were clear the President was going to respond to North Korea's threats following the sanctions with a strong message in no uncertain terms."

The President went into the meeting on the opioid epidemic expected to be asked about the rising nuclear threat from North Korea, but he was not reading from anything when he threatened the rogue nation with "fire and fury," three officials said. The paper in front of him contained notes about the opioid epidemic, not North Korea.

The tone of rhetoric Trump used, which has reverberated around the world and raised alarm among national security and foreign policy experts, did not surprise many around him, the officials said. The President has used nearly that same language in private, the officials said.

White House chief of staff John Kelly was "aware Trump would take a strong tone on North Korea," one official told CNN. He "certainly was not surprised or caught off-guard," the official added.

Some White House aides have trumpeted the President's forceful response to the North Korea.

"This is analogous to the Cuban missile crisis," Trump aide Sebastian Gorka said in an interview with Fox News on Wednesday. "He is saying don't test America, and don't test Donald J. Trump. We are not just the superpower. We were a superpower. We are now a hyper power, nobody in the world, especially not North Korea, comes close to challenging our military capabilities."